Specialists offer advice on management of Palmer amaranth

Two Purdue Extension specialists have developed an information guide to help crop producers across Indiana deal with Palmer amaranth, an aggressive, herbicide-resistant weed.

Weed scientists Bill Johnson and Travis Legleiter released a new publication, Palmer Amaranth Biology, Identification and Management in Indiana, to help farmers control the weed's spread.

"Our publication gives producers a broad view of the weed, and it gives them a starting point to build a preventative program," Legleiter said. "We're letting producers know that if they do encounter the weed, they need to have a proactive plan."

The publication can be found online at https://ag.purdue.edu/btny/weedscience/Documents/Palmer_Bio_Id_Mngmt_pg.pdf

Most populations of Palmer amaranth are resistant to glyphosate, making it very hard to control. The weed has infested cotton and soybean farms in the southern United States in recent years, causing economic hardships for farmers.

Johnson said Palmer amaranth populations were confirmed in the Evansville area in 2011 and in five counties in northwestern Indiana last fall. It is suspected that seeds were present in manure spread over fields that contained cottonseed hulls from the South.

"This weed is arguably the biggest weed problem to hit agronomic crops in the last 10 years," he said. "Very severe yield losses are possible."

Palmer amaranth is a hairless, flowering weed with multiple seed heads. The fast-growing green weed has one female terminal head that can produce 500,000 seeds.

According to Legleiter, the weed can grow 2 inches a day and up to 10 feet tall.

"It can add to the seed bank very aggressively, and the seeds will survive for several years," he said.

Legleiter recommended that farmers develop a plan to control Palmer amaranth and to scout fields for the weed during their growing seasons.

"If you let one or two plants get by in one year, in a couple years you'll have a pretty big issue," he said.

Palmer amaranth was misidentified as waterhemp in Indiana, and Legleiter said that led to mismanagement. Unlike waterhemp, the Palmer amaranth petiole, or the stalk that joins the leaf and main stem, will be as long or longer than the leaf itself.

In their publication, Johnson and Legleiter cover cultural management options, such as crop rotation, tillage and hand-weeding.

"If you do have a few escapes of the weed that are manageable to pull by hand, do it," Legleiter said. "It's going to save you in the long run."

The publication also includes tips and tables on using specific herbicides for pre- and post-emergence control.

Johnson said Palmer Amaranth damage likely will be much more severe for soybean fields than corn fields because there are fewer herbicide options for soybeans.